The methodology disclosed herein generally relates to systems and methods for producing composite laminates using tow placement technology. In particular, the methodology disclosed herein relates to the automated design of variable-stiffness composites comprising plies with spatially varying fiber orientation.
Fiber-reinforced composite materials comprise fibers embedded in a matrix material, such as thermoset and thermoplastic polymer resins. The fibers carry loads and provide strength and stiffness. A tape layer in a composite material has high strength and stiffness in the direction of the fiber, and lower strength and stiffness in a direction perpendicular to the fiber.
Fiber-reinforced composite laminates are usually constructed of plies with constant fiber orientations. Laminate stiffness is varied on a panel-by-panel basis by dropping and adding plies. A more natural way of varying laminate stiffness is to gradually change the fiber orientation in the plane of individual plies. Steered-fiber laminates provide potential to reduce weight, because they allow in-plane stiffness tailoring, resulting in beneficial load redistribution. For certain applications the stiffness can be varied to tailor load paths such that local load levels near critical areas such as cutouts are reduced, ultimately leading to lighter structures. Production of these laminates is possible using advanced fiber placement technology, which enables the placement of curved fiber courses on a surface.
Advanced fiber placement (also known as “tow placement technology”) is a fully automated process for the production of composite laminates that combines the differential payout capability of filament winding and the compaction and cut-restart capabilities of automated tape laying. A variety of machines exist that can deposit different kinds of materials: fiber-reinforced thermoset prepreg (pre-impregnated) materials, fiber-reinforced thermoplastic materials, or dry fibers. Carbon fibers pre-impregnated with thermoset resin are most commonly used in the aerospace industry and therefore the fiber placement process will be described herein assuming a thermoset material system.
Most fiber placement systems have seven axes of motion and are computer controlled. The axes of motion, i.e., three position axes, three rotation axes and an axis to rotate the work mandrel, provide the fiber placement machine flexibility to position the fiber placement head onto the part surface, enabling the production of complicated composite parts. During the fiber placement process, tows of slit prepreg tape are placed on the surface in bands of parallel fibers, called courses (i.e., each course consists of multiple parallel tows). Typical tow widths are 3.175, 6.35, and 12.7 mm (⅛, ¼, and ½ inch). This technique allows fibers to be curved and tows to be cut and restarted individually, making it possible to manufacture parts that are close to their final shape, thus reducing scrap rates. The tow cut and restart capability of fiber placement machines also enables variation of the course width, which can be used to eliminate gaps or overlaps between neighboring courses that are caused by geometry and steered fiber courses.
Advanced fiber placement has substantially increased the capabilities for manufacturing composite laminates, but it also has a number of limitations, which are more important for steered-fiber laminates than for non-steered-fiber laminates. In steered-fiber laminates courses are not parallel to each other, causing courses to overlap each other if the course width is kept constant, assuming no gaps between courses is allowed. Tows can be cut and restarted if overlaps are undesired. The exact position of these tow cuts/restarts with respect to the boundary of a neighboring course or a ply boundary is determined by the coverage parameter. Tows are cut perpendicular to the fiber direction, causing a non-smooth course boundary and small triangular overlaps or gaps. More tow cutting/restarting is needed for steered-fiber laminates than for non-steered-fiber laminates, increasing the influence of these small triangular overlaps and gaps on the laminate quality. Therefore the coverage parameter is more important for steered-fiber laminates. The amount of tow cutting/restarting that is necessary to achieve a “near-constant” thickness can be quantified by the amount of thickness build-up that would occur if no tows would be cut at all, i.e., if the course width was kept constant. Thickness build-up depends on the surface geometry and the amount of fiber steering used to tailor laminate stiffness, but is typically more pronounced in steered-fiber laminates. A physical restriction of fiber placement machines is the minimum length of a tow that can be placed, called the “minimum cut length”. Short tows in non-steered laminates typically occur at ply boundaries, where they are either extended beyond the minimum cut length, or left out. In steered-fiber laminates short tows can occur in the middle of a ply, where extending or leaving out the tow is undesirable, because they create flaws in the laminate. Other effects directly related to fiber steering are tow puckering, folded tows, and straightening of curved tows when they are cut. These effects are more pronounced for smaller in-plane turning radii. A minimum turning radius is defined to ensure laminate quality is not significantly affected by tow puckering or folded tows. Tow straightening depends on lay-down direction and turning radius. Finally, plies in non-steered laminates are usually programmed based on one reference direction and offsets to that curve, or on a small number of guide curves in-between which interpolation is used to define the course centerlines. Steered-fiber laminates require a large number of guide curves to ensure that the as-manufactured fiber orientations match the as-designed fiber orientations. The fiber placement definition, which includes centerline locations, tow cut/restart locations and laydown direction, can be optimized to avoid or minimize the effects of fiber steering and tow cutting/restarting while ensuring the desired fiber orientations are achieved.
There is a need for software capable of optimally translating steered-fiber laminate ply definitions in fiber placement code for manufacturing steered-fiber laminates, taking into account all manufacturing constraints, especially ones that are specific to steered-fiber plies, such as minimum cut length occurring within a ply instead of at the boundary, fiber straightening, and long gaps that might occur when courses go parallel.